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Hi,
I am currently working with a Dimension FastScan system, and have been getting some promising results using carbon nanotube AFM probes, overall I'm really impressed with the instrument.
My question relates to the laser interference I've been observing, and the issues that is causes when trying to engage very softly with a probe.
When engaging in peakforce tapping mode, I have been using the settings below:
engage int. gain = 1
peakforce engage setpoint = reduced to 0.05V which results in a false engage and then increased gradually until the tip engages.
The problem I am having is that the false engage often seems to be a result of the laser interference, and for some probes this can be quite large. This results in having to increase the engage setpoint, which suggests that the engage will not be as gentle.
My understanding is that this interference comes from spillage over the edge of the cantilever and reflection off the surface (worse for reflective samples), which is understandable for instruments with large spot sizes. The fastscan however, has a spot size that is much smaller than the width of the cantilever, does this imply that there must be some transmission through the cantilever?
The image below shows fairly typical interference, however in some cases we observe very large and strange interference (not sinusiodal) which makes engagement and imaging quite difficult.
I commonly use ScanAsyst or FESP/FMV probes, and often I am imaging nanoparticles on silicon (quite reflective).
I was curious if there is anything I can do to minimise this noise such as laser spot position etc...
Regards,
Ash
Hi Ash,
The laser interference is definitely influenced by the presence of a metal coating on the back of the probe and even on the thickness of that coating since we are typically working with very thin coatings. Some light may be passing through the cantilever. The probes that you mention are coated, so you are already using probes that have reduced transmission compared to the uncoated probes.
A few other things that you can try:
I hope that helps!
--Bede
Hi Bede,
Thanks for your reply,
For the first point, this shouldn't be an issue as the optics are always well-focused of the cantilever before approach. Following on from this though, does the small size of the laser spot contribute to the issue of transmission through the cantilever? Would it actually be better to use a spot as large as possible without it "spilling" over the edge of the lever? (maybe intentionally defocusing the beam slightly to increase the spot size, although this would interfere with the engage distance)
Moving the spot to the right seems to help, however my biggest concern is keeping the tip-sample forces as low as possible. Generally I try to keep the spot as close to the tip as possible without losing laser signal, so that the deflection sensitivity is high and the tip-sample forces can be reduced as much as possible.
Also, with regards to the third point; when using CNT probes I do always set the initial scan size to zero. As you said, excessive lateral forces are likely to be more damaging than forces normal to the surface; however I was hoping to reduce both as much as possible due to the delicate nature of the probe.
By using a larger spot, you will have higher sensitivity. This certainly could result in less interference noise after conversion to nm. I would try the recommended spot sizes for your probes (I think it is Large in both cases that you mention). I would not recommend intentially defocusing the laser.